The invention relates to fluid sensing gauges. A common type of gauge is the float gauge, which has a float that floats on the surface of the fluid being measured. The float usually is connected with other members which move with the float as the fluid level changes. Movement of the float and attached members is sensed by a gauge, typically through a magnetic coupling, to provide an indication, either visual or otherwise, of the fluid level.
A common float gauge used for many years incorporates a variable resistor in the gauge to vary a resistance with a change in the fluid level. In such a gauge, a wiper arm is moved along a resistance element as the float changes level. The resistance is measured between one end of the resistance element and the wiper arm, which corresponds to the float position. While such a circuit is effective, it does suffer certain disadvantages. These disadvantages include the requirement for adjusting the resistance output to correspond to the float position. Further, changes in such variables as contact resistance, hook-up wire length, temperature, and voltage applied across the resistance can actually vary the response of the gauge, leading to an inability to reproduce results and maintain gauge accuracy.
Some of the disadvantages of the variable resistor gauge were addressed in U.S. Pat. No. 4,911,011, which disclosed a gauge with a magnetically driven voltage divider. However, gauges with the voltage divider disclosed in that patent provide a high impedance output signal which is a percentage of the input voltage. Thus, the gauge has the disadvantage of having limited applications unless the user provides additional components to employ the electrical output signal of the gauge.
Thus, there has been a need for a gauge which is more universally acceptable and readily received by standard equipment, such as refrigeration units and tow motors manufactured by a number of different manufacturers. The gauge of the present invention has a compact design, provides an output which is more immune to noise and has circuitry which is less vulnerable to environmental hazards. The present invention also has the advantage of being useful with different power systems, such as 12 volt DC or 24 volt DC systems found on many industrial vehicles and apparatus. The present gauge accepts power from any such system and provides an industry standard output signal which will interface directly with industrial control devices and meters.